Simultaneous refrigeration failures at two fertility clinics in San Francisco and suburban Cleveland have damaged or destroyed potentially thousands of frozen eggs and embryos in the biggest such loss on record in the U.S. The malfunctions have left parents-to-be heartbroken and baffled experts.

Here are some questions and answers about the two cases.

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WHAT HAPPENED?

In Ohio, University Hospitals Cleveland Medical Center estimates 2,000 eggs and embryos may have been damaged or destroyed when an unexplained storage tank malfunction caused temperatures to rise on March 4. The medical center apologized.

On the same day in San Francisco, an embryologist at the Pacific Fertility Clinic noticed the liquid nitrogen level in one tank was very low during a routine check, clinic president Dr. Carl Herbert told ABC. He said the tank was immediately replenished and the embryos were later transferred to a new tank.

There's no known connection between the two episodes, said Dr. Kevin Doody, lab director at the Center for Assisted Reproduction in Texas and past president of the Society for Assisted Reproductive Technology. He said such breakdowns are extremely rare, and two at once is "beyond stunning."

"It's two black swan events happening in the same day. One of them causes the beehive to buzz. Two? We're all in shock," Doody said.

Officials have yet to say exactly what went wrong.

Barbara Collura, president of the patient advocacy group RESOLVE, called for "a very open, transparent investigation where the results are clear and public for all of us."

"We all need to know what has happened," she said.

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CAN THE EMBRYOS BE USED?

Scientists can easily tell by looking through a microscope whether an egg or embryo survived a thaw, said David Ball, another past president of the Society for Assisted Reproductive Technology.

If doctors were to implant a damaged embryo, it might not lead to a pregnancy, but if it did, it would not raise the danger of birth defects in the child, Ball said.

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WHO WILL INVESTIGATE?

The American Society for Reproductive Medicine said it plans to review the incidents with the clinics and their equipment suppliers this week. The group will then make recommendations to its members.

"In the meantime, infertility clinics around the country have been double- and triple-checking their own procedures and equipment to ensure everything is working properly," the group said in a statement.

Accrediting bodies such as the College of American Pathology may also conduct reviews, and the clinics themselves will investigate or hire outside experts to do so, Doody said.

But government review is unlikely because there is minimal federal oversight of fertility clinics. The politics surrounding abortion have made federal regulation too tricky, said George Annas, a medical ethicist at the Boston University School of Public Health.

"We've never been able to separate the embryo debate from the abortion debate in the United States," Annas said.

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WHAT RECOURSE DO PATIENTS HAVE?

At least two lawsuits have been filed against the Cleveland-area hospital by couples who were trying to conceive.

The patients will have to prove negligence, Annas said. "Nobody's going to be charging these clinics with murder or manslaughter," he said.

Determining any damages owed to the patients could involve looking at the cost of repeating a fertility treatment, which can run up to $15,000, he said, "unless it's the last embryo you could make because one partner is dead."

"It's hard to think it's worth more than the cost of making more embryos, unless you believe these are babies," he said. "Then it's hard to put a monetary value on it because it's so high."